Elastomeric sealing assemblies are commonly used in oil and gas well downhole operations, to sealingly isolate a section of a flow bore, either in the tubing, or in the annulus between the tubing and the casing. For example, sealing assemblies are used with such downhole equipment as packers (which seal the annulus) and bridge plugs (which seal the tubing). Sealing assemblies seal by providing a solid ring of elastomer, known as a sealing element, that can extend from the setting tool to the inner wall of the flow bore, the inner wall acting as a setting surface.
Currently there are two main methods by which sealing assemblies are set. The first is by use of a propping sleeve, wherein the sealing element sits on a sleeve of varying diameter. While the tool is being run into the hole, the sealing element sits on a reduced diameter portion of the sleeve, so that the sealing element is retracted and does not interfere with the travel of the tool into the hole. Once the tool is in the proper position in the hole, the sealing assembly is set by forcibly sliding the sealing element onto a larger diameter portion of the sleeve. Expanded, the sealing element contacts the wall of the flow bore, preventing fluids from traveling through that flow bore. When the tool is to be released, the sealing element is moved back onto a smaller diameter portion of the sleeve.
The other method of setting a sealing assembly is by compression. For this method, the sealing assembly is positioned between two hard end surfaces. To set the sealing assembly, the end plates are moved towards each other, longitudinally compressing the sealing element, thereby forcing the element to expand radially outward towards the setting surface. To release the sealing assembly, the plates are moved back apart, and the sealing element hopefully relaxes and returns to an approximation of its original, smaller diameter shape.
Two problems have arisen and are associated with compressive sealing assemblies. First, to insure a complete and strong seal, the sealing element must uniformly and forcefully contact the sealing surface. In some cases, due to the radial outward distance the sealing element has to travel, it may not contact the sealing surface at all, or the entire perimeter of the sealing element may not contact the sealing surface. Also, if the sealing element has to travel a long radial distance to reach the sealing surface, the sealing element can distort and twist, causing an imperfect seal. Further, even if the sealing element reaches the sealing surface, it may not be pushed forcibly against the sealing surface, making it easier for leaks to develop when a sufficient pressure differential is experienced across the sealing element.
The second problem is that after the sealing element has been in its extended and set position for a sufficient length of time, the elastomer begins to permanently distort, such that when the end surfaces are moved back apart, the sealing element will retract little or not at all. The tool then becomes much more difficult to remove from the hole, as the sealing assembly tends to drag on the setting surface as well as hanging up in any reduced diameter portions of the flow bore.
Hence, what is needed is a compressive sealing assembly that can be forcefully and uniformly contacted against the sealing surface to preventing leaks, as well as being able to be forcefully retracted away from the sealing surface, to make removal of the tool easier.